1. Field of the Invention
The present invention relates to methods and devices for tissue stimulations, and more specifically to such methods and devices mimicking natural biological stimulation of tissues.
2. Description of the Prior Art
Artificial activation or stimulation of tissues has been known for centuries. For example, pacemakers, implantable cardioverters and implantable defibrillators are today used for stimulating the heart muscle by applying a current or potential pulse. Such an electrical stimulation pulse is transferred to the heart tissue by an implantable electrode for treating various arrhythmias, such as bradycardia.
Although electrical pacing is readily used and works fairly well it is not a biologically optimal method. The electrode used in the electrical pacing needs a surface potential of at least fifty or even several hundreds times as large as the natural potential needed to induce stimulation. High electrode surface potential in the aqueous biological environment in the patient body is associated with several drawbacks. For example, conventional electrical pacing causes generation of undesirable irreversible reaction products at the electrode-electrolyte interface. In this interface, redox processes involving, among others, proteins in the host body results in reaction products that can have an inflammatory effect and thereby contribute to a local inflammation at the site of the electrode. It is even possible to have gas development and oxidation and release of electrode material, which will cause inflammation and other deleterious conditions in the host body. For example, electrical pacing at potentials below about −0.8 V vs. Ag/AgCl (sat. KCl) causes development of hydrogen gas at the electrode (H2O+2e−→H2+2OH−). In addition, the produced hydroxide ions will change the local pH outside the electrode, which have negative effects on the surrounding tissue. Furthermore, depending on the electrode material employed, the produced hydrogen can be absorbed by the electrode. For example, if the electrode comprises a titanium substrate, the absorbed hydrogen will cause brittleness, possibly resulting in crack formation and flaking of the electrode coating. A further negative reaction at the pacing electrode is the production of hydrogen peroxide and hydroxide ions (O2+2H2O+2e−→H2O2+2OH−). The hydroxide ions negatively affect the local pH and the hydrogen peroxide is a strong oxidizing agent that can damage biological molecules and cells.
There is therefore a need for a more biologically attractive technique for stimulating tissue that is not marred by the problems and disadvantages of electrical pacing described in the foregoing.